Ion implantation has become a standard accepted technology of industry to dope workpieces such as silicon wafers or glass substrates with impurities in the large scale manufacture of items such as integrated circuits and flat panel displays. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece so that they are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Ion energy is used to control junction depth in semiconductor devices. The energy levels of the ions that make up the ion beam determine the degree of depth of the implanted ions. High energy processes such as those used to form retrograde wells in semiconductor devices require implants of up to a few million electron volts (MeV), while shallow junctions may only demand energies below 1 thousand electron volts (1 keV).
A typical ion implanter comprises three sections or subsystems: (i) a terminal for outputting an ion beam, (ii) a beamline for mass resolving and adjusting the focus and energy level of the ion beam, and (iii) a target chamber which contains the semiconductor wafer to be implanted by the ion beam. The continuing trend to smaller and smaller semiconductor devices requires a beamline construction which serves to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy levels permit shallow implants. Source/drain junctions in semiconductor devices, for example, require such a high current, low energy application.
Low energy ion beams which propagate through a given beamline construction suffer from a condition known as beam "blow-up", which refers to the tendency for like-charged ions within the ion beam to mutually repel each other. Such mutual repulsion causes a beam of otherwise desired shape to diverge away from an intended beamline path. Because the problem of beam blow-up increases with increasing beamline lengths, a design objective of preferred beamline constructions is to minimize or shorten the length of the beamline.
Typically, the target chamber is oriented generally perpendicularly with respect to the axis of the shortened beamline so that the ion beam strikes normal to the plane of the substrate. However, certain implants require the ion beam to strike the substrate at an orientation several degrees from normal. In order to permit such implants, the target chamber is made pivotable about the axis of the beam path. For example, a tilt-twist mechanism may be provided to allow pivoting in each of two perpendicular axes that generally lie in the plane of a substrate in the target chamber. An expansible bellows provides the interface between the beamline and the movable target chamber.
For applications where the bellows is required to move in simple axial compression or extension, no lateral forces are present, and the bellows corrugations can adequately handle the extensive or compressive forces in the axial direction. However, when the target chamber pivots with respect to the beamline path, the bellows typically experience shear forces in the plane perpendicular to the beam path. The bellows mounting is urged laterally within this plane (i.e., the bellows mounting tends to undergo a lateral offset). Even small lateral movements in metal welded bellows may cause large shear stresses at the mounting locations.
Fixedly mounting the bellows on both ends focuses these shear stresses in the plane perpendicular to the beam path (and parallel planes) at the locations of the fixed mountings. This shear stress may result in premature failure of the bellows by reducing the number of cycles in its lifetime. Because the implantation process is typically performed in a high vacuum (e.g., down to 1.times.10.sup.-7 torr) process chamber to prevent dispersion of the ion beam and minimize the risk of contamination of the substrate by airborne particulates, any breach in the integrity of the bellows will result in loss of this vacuum condition. The loss of vacuum and the resulting contamination of the interior of the bellows will compromise the implantation process being performed.
It is an object of the present invention, then, to provide a means for alleviating the shear stress in a vacuum bellows. It is a further object of the present invention to provide an improved bellows for connecting two portions of an ion implanter that move with respect to each other. It is yet a further object of the invention to provide a lateral stress relief mechanism for a vacuum bellows, including a bellows for use in an ion implanter.